Awesome

This is a new MicroPython driver for the BME680 temperature, humidity, pressure and VOC sensor. The driver was written from scratch in order to try overcoming the issues I encountered with existing drivers, after testing and reading the code of existing drivers and other C-coded drivers available in the wild.

Since this is a complete rewrite, I prefixed the driver name with "az" to make sure that it is easy to distinguish from the many forks for MicroPython / CircuitPython I found of variations and fixes of the same driver.

API

Creating the BME680 object

Just pass an i2c object as first and only argument to the constructor.

i2c = SoftI2C(scl=Pin(5),sda=Pin(16))
bme = BME680(i2c)

The i2c object needs to support the following methods:

i2c.readfrom_mem()
i2c.writeto_mem()

Configuration of parameters like oversampling, IIR filter and what reading to take are all performed via the other APIs and not via initialization.

Obtaining measures

This driver is conceived to take measures in two step. The first call is used in order to actually set the BME680 in "forced" mode and obtain an analogical reading of the selected channels (temperature, pressure, humidity, gas).

bme.measure()   # Obtain temp, humidity, pressure from the chip.

By default, gas reading is not performed. To read the VOC/gas resistance value, in ohm, the chip requires to heat the reacting surface, wait some time and so forth. This process is both slow and expensive in terms of consumed currnet, so if you want this reading, use:

bme.measure(gas=True)

Then the conversion from the raw data to the actual measure is obtained using the following set of APIs:

bme.temperature()       # Temperature in celsius
bme.pressure()          # Pressure in hPascal
bme.humidity()          # Relative humidity
bme.gas()               # Gas sensor resistance in ohms

All the above calls return just the value, but .gas() which returns a dictionary like this:

{'ohms': 225257, 'valid': True}

This is required because sometimes the chip will tell us that the reading is invalid, as there were issues with heating the VOC-sensitive surface and so forth.

You can also obtain the estimated location altitude in meters or feet:

bme.measure()
p = bme.pressure()
bme.altitude(p)

# Or, for a better approximation, provide the
# pressure at sea level.
bme.altitude(p, sea_level_p = 1013.25)

Changing oversampling and filter

For more precise readings, the BME680 performs optional oversampling of temperature, pressure and humidity values. You can select different oversampling values for each of the channels.

Valid oversampling values are: 0, 1, 2, 4, 8, 16. A value of zero means to skip the measure completely. Greater oversampling values correspond to greater measure delay but lower noise.

For example the following measure will be performed selecting only the temperature, with an oversampling value of 8.

bme.measure(gas=False,p_os=0,t_os=8,h_os=0)
print(bme.temperature())

It is also possible to select and change settings for the IIR (infinite impluse response) filter. The IIR filter is used in order to smooth the readings of temperature and pressure, that are subject to strong noise. A value of 0 totally switches off the IIR filter.

Valid filter coefficient settings are: 0, 1, 3, 7, 15, 31, 63, 127. A higher filter coefficient means more aggressive filtering.

For instance if I want unfiltered pressure data, I'll use:

bme.measure(gas=False,p_os=1,t_os=0,h_os=0,iir_filter=0)
print(bme.pressure())

Changing VOC reading parameters

The chip is already kinda magic enough, thanks to Bosch(TM) providing only a closed source library for transforming the raw gas ohm readings to an air quality index. Moreover it is possible to select the heating temperature of the gas detecting surface and the milliseconds we will wait in order to read the value.

I believe it's better to stick with the parameters everybody seems to use:

320 degrees, 150 milliseconds

However if you really want to play with this, use:

bme.measure(gas=True,gas_temp=350,gas_ms=200)

Temperatures range from 200 to 400 degrees. Heating-on times up to a bit more than 4000 milliseconds. See the set_gas_heater() method in the code for details and make sure to check the datasheet.

Examples

The example.py file is just a very simple example that shows how to setup the i2c object and use it to create the BMP680 object. It will print all the available readings using the default oversampling and filtering.

The door_detector.py example shows how to get unfiltered data with less delay, in order to detect if somebody is opening/closing a door in your room.